1. Field of the Invention
The invention relates to apparatus for scanning a record carrier having substantially parallel tracks, which apparatus comprises a closed-loop position control system with position and velocity feedback for maintaining the scanning point centered on a specific track. Such control system includes a positioning system for moving the scanning point in a direction transverse to the tracks, a measuring system for generating a velocity signal indicative of the velocity with which the scanning point is moved in a direction transverse to the tracks and an error signal which, within a predetermined limited range to either side of the center of said specific track, is substantially proportional to the deviation between the track centre and the scanning point; and a control circuit for controlling the positioning system in accordance with the error signal and the velocity signal.
The invention further relates to a moved form of the aforesaid control circuit.
2. Description of the Related Art
Apparatus of the aforesaid kind is known from Applicant's U.S. Pat. No. 4,037,252.
Such patent describes an optical scanning system for reading an optically readable disc-shaped record carrier. The error signal derived in such scanning system is a radial-error signal which during a movement of the scanning point varies periodically at a frequency corresponding to a number of tracks traversed per unit of time, and which within a range of 1/4 of the track pitch to either side of the track center is substantially proportional to the deviation between the scanning point and the track centre. Moreover, such system derives a track-loss signal which indicates whether the scanning point is positioned substantially on the track or substantially midway between two tracks. The velocity signal is derived from the radial-error signal and the track-loss signal. The velocity signal and the radial-error signal thus obtained are applied to the control circuit to derive a control signal for controlling the positioning system depending on the aforesaid signals. This yields a tracking control system with velocity feedback.
The drawback of such known scanning system is that in the case of a deviation larger than half the track pitch the sign of the error signal will no longer correspond to the sign of the deviation. If as a result of a disturbance, in particular a mechanical shock, the tracking error becomes larger than half the track pitch, the scanning point will be controlled towards an adjacent track instead of the original track. This is troublesome, in particular when the scanning system is used in an environment where it is frequently exposed to mechanical shocks, as for example in portable audio equipment or in automotive uses. In such cases mechanical shocks will frequently result in the scanning point being moved off the track to be followed.